Hardboard pressing methods and apparatus



Oct. 3, 1967 R. w. DIBDIN 3 HARDBOARD PRESSING METHODS AND APPARATUS Filed Nov. 15, 1963 3 SheetS Sheet 1 HA Tf 0F THICK/V555 HEDUCT/ON PRESS/[V0 TIME Rosam- W. DIED/ Oct. 3, 1967 R. W. DlBDlN HARDBOARD PRESSING METHODS AND APPARATUS Filed Nov. 15, 1963 3 Sheets-Sheet 2 R. W. DlBDlN HARDBOARD PRESSING Filed Nov. 15, 1963 METHODS AND APPARATUS 5 Sheets-Sheet 3 United States Patent ABSTRACT OF THE DISCLOSURE A method of manufacturing hardboard sheets of predetermined moisture content based on the discovery that the drying of hardboard under pressure is accompanied by a reduction in thickness and that the rate of reduction in thickness is proportional to the rate of drying and thus the moisture content of the board- The method includes the steps of measuring the thickness reduction within -a predetermined time during pressing in which the rate of thickness reduction is markedly decelerated, and controlling the pressing operation dependently thereon. For example, the measurements of thickness reduction are converted into electric pulses of a duration proportional to the rate of thickness reduction, and these pulses control the pressing operation. The corresponding apparatus produces electric pulses of varying duration propor-,

tional to the rate of thickness reduction, and includes an electronic time-delay relay which operates the press control only when any of the electric pulsesexceeds a predetermined duration.

This invention relates to hardboard pressing methods and apparatus and more specifically to the control of the moisture content of such pressing cycle. 7 1

It is recognized in the hardboard industry that the drying time for hardboard sheets, which determines their moisture content varies considerably with the variation in pressing temperature, initial pressure, application time of the initial high pressure andthe level of final pressures used. 7 v

The drawing time is further dependent, amongst other factors, upon the kind of timber used as a starting material in the hardboard manufacture, the type of fibers prepared during processing, the density ofthe board mate"- rial and the thickness of the final board.

Owing to all these variable factors it has not been possible to predetermine accurately the pressing tirne required for any given moisture content and thus the pressing and drying time has generally been extended to obviate random batches of undried or delaminated boards. Consequently, the majority of batches of the pressed board are overdried to a varying and uncontrolled degree and the productivity of the production plant is reduced-by the, excess time taken for the .drying process.

It is obvious that the quality of the produced board will, therefore, vary with regard to its moisture content and resistance and to its internal bonding.

It is an object of the invention to overcome the abovev difficulties by an improved manufacturing method and apparatus which permits the manufacture of hardboard in a press under conditions of heat and pressure with a precise, predetermined moisture content or a controlled degree of overdrying if so desired.

The invention is based on the discovery that the drying of hardboard under pressure is accompanied by a reduction in the thickness and that the rate of reduction in 3,345,441 Patented Oct. 3, 1967 ice . pending on a predetermined rate of thickness reduction.

This value then determines a specific moisture content oi the pressed hardboard.

It has been found that the rate of thickness reduction and the rate of drying remain fairly constant until the moisture content falls to about 10 percent. Below this value the rate of thickness reduction falls suddenly and markedly until the moisture content approaches about 1 percent.

This latter range includes the moisture contents normally required in hardboard manufacture. Owing to the sudden changes in the rate of thickness reduction over a predetermined time a very precise gauging of the correct pressing or drying time to obtain the required moisture content is made possible.

Experiments have shown that the greatest accuracy is achieved for moisture contents between 8 percent and 2 percent whilst below this value the rate of thickness reduction approaches zero.

In some cases an overdried board is required. This can be readily achieved with the above method by introducing a time delay in the action of the press release. In this way it is possible to accurately predetermine and control the amount of overdrying. I

boards at the completion of the V The above described method operates independently of the actual thickness of the hardboard normally within the range of to thick. The new method reduces the average time of pressing by 10 percent to 25 percent and at the. same time reduces significantly variations in board quality.

The invention will be described hereinafter in more detail by way of example in connection with the drawings, in which: 7

'FIGURE 1 is a diagram showing the relation between pressing time and thickness reduction in the manufacture of hardboard sheets;

- FIGURE 2.is a block diagram of the apparatus to, carry out the method according to the invention;

, -.FIGURE 3 shows details of a microswitch arrangement to convert thickness reductions of the hardboard sheet during pressing into electric pulses;

FIGURE 4 is a circuit diagram of a known electronic time-delay relay responsive to pulses exceeding a predetermined duration, and

' FIGURE 5 shows a circuit diagram of a known press control unit including a timing device.

'In FIGURE 1 the curve 1 shows the relation betweenthe rate 'of reduction in thickness of a hardboard sheet during pressing and the pressing time. As can be seen the rate of thickness reduction remains fairly constant until a point 2 is reached at which the residual moisture content is approximately 10 percent. On further pressing the rate .of thickness reduction falls sharply in accordance.

with the reduction in residual moisture content as shown, forexample by points 3, 4, Sand 6 designating moisture contents of approximately 4 /2 3 /2 2 /2 and 1% respectively. Below point 6, i.e., when the residualmoisture content has fallen below about 1 /2% the curve flattens out again to a very much reduced rate of thickness reduction.

The beforementioned change in the rate of thickness reduction is used to control the moisture content of a hardboard sheet during pressing by an arrangement as,

" shown schematically in FIGURE 2.

Changes in the thickness of the board under pressure ."e indicated by an electrical microswitch 7 which is temorarily operated by the moving press table 8 once in ich .055" of travel of the table.

This linear movement of the press table 8 is converted to rotary movement of a rnultilobe cam or ratchet bed 9 by means of the moving table 8 actuating a lever l) which is connected to the input shaft of an amplifyi-g gear train or gearbox 11. The rnultilobe cam 9 opertesthe microswitch 7 giving on-off electric impulses the uration of which depends upon the rate of movement f the press table 8 which is governed by the moisture ontent.

The microswitch 7 is the initiating switch of a standard lectronic time-delay relay 12. Each time the initiating witch 7 opens in less than the time preset on the timelelay relay 12 the latter resets to approximately zero. Fhus until the press movement reaches a certain rate herelay cannot operate. When the initiating switch 7 remains closed for a period in excess of the preset time he time-delay rel-ay operates and either opens the press ia the normal automatic controls 13 and the press conrol valve14- or provides a signal to indicate the press :ycle is complete and the press should be opened.

It overdrying is desired a timing device is incorporated n the normal press control 13 and operated either auton-atically or manually.

This. period of overdrying can be accurately controlled ;ince it commences at a predetermined constant moisture :ontent and proceeds for a predetermined constant time.

The arrangement converting the thickness changes into electric pulses is shown in more detail in FIGURE 3. The press table 8, when moving upwards during the pressing operation to press the sheet 19, moves via an extension 15 the lever in counterclockwise direction as the latter is pivoted within a gear box 11 by means of a shaft 16. This shaft carries a gear wheel 20 which meshes with the gear wheel 21 on a shaft 22, the ratio being such that the movement of lever 10 is amplified. Further gear trains are provided according to the required amplification and the last shaft 17 carries the camor ratchetwheel 9, which rotates in the direction of the arrow.

A microswitch 7 has its operating arm 18 in contact with the teeth of ratchet wheel 9 so that each time the arm 18 rides over one of the teeth the contact of switch 7 is closed, but open again as soon as the arm 18 falls into a valley between two teeth. Thus the duration of each pulse. produced by closing the contacts of switch 7 is dependent on the amount of movement produced by press table 8 within a predetermined time, i.e. the rate of thicknes reduction of the pressed sheet, which in'turn is an indication of the residual moisture content of the board.

The circuit of the electronic time-delay relay 12 is shown in FIGURE 4. A thermionic valve V has its filament connected to an AC power supply via a transformer TR1. The anode is connected over a relay CR1 with the power supply, while the cathode. is connected with the power supply over the external microswitch 7. The grid of valve V is connected over resistor R1 with parallel condenser C1 to a potentiometer P which is connected in. series with a resistor R3 across the power supply.

When the microswitch 7 is open, the condenser C1 is charged with the polarity as indicated, the current flowing from terminal N over potentiometer P and its slider to condenser C1 and from there through valve V and resistor R2 to terminal A. The grid and cothode of the valve act in this case as a rectifier.

No current flows through relay CR1 and its contactcr1 remains open.

When the microswitch 7 is closed by the operation of the press as mentioned above, the cathode of valve V is. directly connected to terminal N, and thus a circuit is prepared. for relay CR1 from terminal A over'relay CR1, Valve V to terminal N. The valve, however, does not yet pass any current, as the grid. is blocked. by the DC voltage across condenser C1, which is greater than the AC voltage from terminal N to the slider of the potentiometer P. The condenser C1, however, starts to discharge over the resistor R1 and as soon as the voltage across the condenser C1 is sufficiently low the valve V will pass current and relay CR1 will operate closing its contact crl. The time delay for the operation of CR1 can be adjusted by shifting the slider of potentiometer P. To prevent any vibration of relay CR1 owing to the alternating current, a condenser C2 is connected in parallel thereto, which discharges through the relay winding during the half-cycles at which the valve V does not pass any current.

When microswitch 7 opens again, the circuit for relay CR1 is interrupted and condenser C1 is charged again. Thus, if microswitch 7 is opened, before condenser C1 is sufiiciently discharged to allow valve V to pass current, relay CR1 remains unoperated. The relay operates only if the microswitch 7 remains closed longer than the time set on potentiometer P for the discharge of condenser C1.

The operation of the press control unit will now be described in connection with FIGURE 5.

After the press has been charged with fibrous mats, the operator presses the start button ST. This energises relay CPR which closes its contact cprl by-passing button ST and providing a holding circuit for relay CPR. At its contact CPIZ a circuit is closed for close-press solenoid CS This moves an hydraulic valve (not shown) from the press-open position to press-close position, and allows the hydraulic medium to close the press.

The hydraulic pressure rises when the press is closed, and, after a predetermined pressure, closes pressure switch PSW and thus energises relay PSWR. This relay opens contact pswrl, releasing press relay CPR, and closes contact pswr2 which energises the cycle clock solenoid CCS, starts the cycle clock motor CCM and the time cycle control motor TCCM via the normally closed contact tccrl.

The time cycle control motor TCCM commences operations and drives a cam shaft operating in accordance. with pre-set times the switches TCC3, TCC-7 and TCC-8. It closes firstly switch TCC-8, which then remains closed until the end of the cycle, by-passing contact pswr2.

The pressing cycle proceeds and after a predetermined time (generally between 45% and of the total cycle) the time cycle control motor TCCM closes switch TCC-3 which energises relay TCCR. This relay opens contact tccrl, thus stopping the cycle clock motor CCM, the time cycle control motor TCCM and at the same time closes contact tccr2 in readiness for the signal from the standard electronic time-delay relay TME, details of which are shown in FIGURE 4.

At approximately this stage the press moving table 8 (FIGURE 3) is actuating the microswitch 7 by means of the arm 15 projecting from the moving press table 8 and engaging the arm 10 on the amplifying gearbox 11.

This gearbox amplifies the gradually decreasing linear movement of the press into rotary movement of the cam 9 as mentioned above to open and close the microswitch 7 at a gradually decreasing rate thus providing timed electrical impulses to the, electronic time-delay relay TME (FIGURE 5). Each time the switch 7 opens, the timedelay relay by virtue of its circuitry resets to approximately zero. Thus when due to a furtherdecrease in the rate of movement of the press table the initiating switch 7 remains on for a period of-time in excess of the preset timer delay, the time-delay, relay completes its cycle, operates and closes contact cr1 which closes a circuit for relay TMER via the previously closed contact tccr2.

The relay TMER provides its own holding circuit over con ract tmez-Z; and r starts he cyc e lo k. motor CCM.

and the time cycle control motor TCCM via contact tmerl.

The press cycle is now complete and control again rests with the time cycle control motor TCCM which closes switch TCC-7 to energise the open-press relay OPR. This relay opens contact oprl to break the circuit for close-press solenoid CS, and closes contact opr2 to energise press-open solenoid OS via normally closed contact cpr3. The solenoid moves the hydraulic valve from the press-close to press-open position and allows the press to open.

The time cycle control motor TCCM continues to operate via switch TCC-S, opens switches TCC-7 and TCC-3 releasing relay TCC2 ready for the next press cycle and stops itself by opening switch TCC-8.

Overdrying if desired may be achieved by repositioning switch TCC-7 on the time cycle cam shaft to give any preset desired time of overdrying. The overdrying period can thus be controlled accurately, as it always starts from a point at which the board has reached a pre-determined moisture content.

From the instant switch TCC-3 stops the time cycle control motor TCCM, the press cycle is controlled solely by operation of the time-delay relay TME dependent on the rate of reduction in moisture content of the fibrous mats which is directly related to the rate of press closing movement.

FIGURE 5 shows one form in which the invention can be superimposed on a known press control arrangement but the invention can be applied to other press control arrangements without in any way materially altering the normal control methods and/or apparatus.

It must be understood that the apparatus described above shows only one form to carry out the method of the invention and modifications can be devised without departing from the scope of the invention.

I claim:

1. A method of manufacturing hardboard sheets under heat and pressure, said method including the steps of determining the required moisture content of the sheet by measuring the thickness reduction within a predetermined time during pressing and by controlling the pressing operation depending on a predetermined rate of thickness reduction.

2. A method according to claim 1 in which the measurements of thickness reduction are converted into pulses of electrical current at a duration proportional to the 6 rate of thickness reduction, said pulses controlling the pressing operation.

3. A method according to claim 2 in which said pulses terminate the pressing operation when their duration exceeds a predetermined value proportional to a predetermined rate of thickness reduction.

4. A method according to claim 3 in which said pulses terminate the pressing operation with a predetermined time delay.

5. Apparatus for determining the required moisture contents of hardboard sheets manufactured in a press including a press table, said apparatus comprising converting means operated by said press to convert the movement of the press table into electric pulses of a duration proportional to the rate of thickness reduction of said sheet produced by said press table, pulse responsive means connected with said converting means and operable by said pulses when the duration of each pulse exceeds a predetermined value and connections from said pulse responsive means to press control means, to control the movement of said press table when a predetermined rate of thickness reduction is reached independent of the actual thickness of the pressed sheet.

6. Apparatus according to claim 5 in which said converting means consist of a microswitch in working relationship with a ratchet wheel and intermittently operated thereby, said ratchet wheel being coupled by a gear train to an operating lever, said operating lever being in engagement with a press arm fixed to said press table, to translate linear movement of said press table into rotating movement of said ratchet wheel.

7. Apparatus according to claim 5 in which the pulse responsive means consists of an electronic time-delay relay, said relay being operable by electric pulses of a duration exceeding a predetermined value, and having means to pre-set said relay to said predetermined value.

8. Apparatus according to claim 5 in which the press control means include a timing device to terminate the pressing operation with a predetermined time delay.

References Cited UNITED STATES PATENTS 2,618,813 11/1952 Patton et al. 18-4 X 2,822,028 4/1958 Himmelhefer et a1. 264 2,956,307 10/ 1960 Fahrai 24440 X 3,026,800 9/1965 Muller 184 WILLIAM J. STEPHENSON, Primary Examiner. 

1. A METHOD OF MANUFACTURING HARDBOARD SHEETS UNDER HEAT AND PRESSURE, SAID METHOD INCLUDING THE STEPS OF DETERMINING THE REQUIRED MOISTURE CONTENT OF THESHEET BY MEASURING THE THICKNESS REDUCTION WITHIN A PREDETERMINED TIME DURING PRESSING AND BY CONTROLLING THE PRESING OPERATION DEPENDING ON A PREDETERMINED RATE OF THICKNESS REDUCTION. 